As indicated above, the present invention relates to the post-treatment of laminated webs which include outer nonwoven non-elastomeric layers between which there is sandwiched a layer of cellulosic fibers to alter the filament spacing and structure of at least the nonwoven non-elastomeric webs. The terms "web" and "layer" are used herein interchangeably at times, the distinction therebetween being obvious from the context in which the terms are used. In either event, "web" and "layer" imply a self-supporting planar member formed of fibers of filaments as the case may be. It should be also observed that the terms "filaments" or "fibers" are used interchangeably herein, although "fibers" in nonwovens generally refers to discontinuous strands and "filaments" as continuous strands. The present invention contemplates webs with continuous synthetic thermoplastic filaments and/or discontinuous fibers in the non-elastomeric nonwoven layers of man-made fibers.
In the present invention, a precursor web made up of an inner layer of staple-length cellulosic fibers which is sandwiched between outer layers of man-made fibers, i.e. synthetic, thermoplastic, nonelastomeric fibers, is consolidated by heating and drawing in the machine direction (MD) to provide a consolidated web which has reduced measures of pore size and enhanced breathability, strength, hand, absorbent capacity, wicking and barrier properties. The layers of man-made fibers preferably are formed by meltblowing or spunbonding techniques. Meltblown fibers of these man-made fibers preferably are of a diameter of between about 0.5 and about 10.0 micrometers; whereas, the diameters of the fibers in spunbond webs overlap with meltblown webs on the low end at about 8.0 micrometers and may range up to 50 micrometers of more on the upper end of their diameter range. Spunbond webs generally are coarser but stronger than meltblown webs because spunbond fibers are given notable orientation after quenching. In either instance, the fibers are formed into self-sustaining webs. The preferred web weight of a meltblown web for use in the present invention is light weight, having a weight in the range of between about 0.05 and about 10 oz/yd.sup.2, and most preferably between about 0.25 and about 2 oz/yd.sup.2. The preferred weight of a spunbonded web for use in the present invention is also light weight having a weight between about 0.1 and about 10 oz/yd.sup.2, and most preferably between about 0.3 and about 2 oz/yd.sup.2. Webs of weights lighter than about 0.05 oz/yd.sup.2 tend to be of insufficient fiber density for containing the cellulosic fibers and providing the strength and other properties desired in the composite web. The heavier weight webs, i.e. above about 10 oz/yd.sup.2 tend to develop undesirably harsh composite webs when combined with the cellulosic fiber layer. More specific descriptions of the spunbonding and meltblowing processes, and the webs so produced are given in the publication entitled: "Proceedings, Fiber Producer Conference 1983", Apr. 12, 13 & 14, 1983, pp. 6-1 through 6-11, such publication being incorporated herein by reference.
Since the development of time meltblowing process by the Naval Research Laboratory in 1951 (published in 1954 by the U.S. Department of Commerce in an article entitled "MANUFACTURE OF SUPERFINE ORGANIC FIBERS"), which publication is incorporated herein by reference, there has been a considerable effort by several companies operating in the industry to find new uses for the nonwoven product having microsized fibers. Because of the random, geometric assembly or structure of the fibers, and relatively small fiber size, the fibers have received extensive use as filters. Further and/or different uses of these meltblown webs is desired.
In the formation process for most random laid or unordered fibrous webs, the pore size that develops is directly related to the square of the fiber diameter. The spunbonded process is distinguished from meltblowing by self-bonding and non uniform draw down (plastic deformation) of filaments forming the web. Thus meltblown webs have a relatively broad distribution of fiber diameters. Typical nonwoven webs produced by meltblowing have fiber diameters of 0.5 to 20 microns, preferably 0.5 to 8 microns, making them suitable for filtering out 5 micron particles at 80 percent efficiency or greater. It is known that filtration can be improved by practicing the web formation process to produce smaller and smaller diameter fibers while controlling other formation parameters such as porosity and thickness. As noted above, this results in smaller pore size thereby improving the efficiency of particle removal in filtration. By operating the meltblowing process under extreme conditions, tile fiber size can be produced in the order of 0.1 to 5 microns. The process, however, has the following disadvantages low production rates high energy usage. In order to improve the properties of the nonwoven web, efforts have been made to post-treat the webs by a variety of processes. Such efforts have included post calendering the web to improve the tensile strength of the web, and post electrification as disclosed in U.S. Pat. No. 4,592,815 to improve filtration performance of the web, to name but two of such efforts. It is significant to note that none of these prior art techniques have been directed specifically at planar consolidation to reduce the size of the pores in the web. Neither is it known to attempt consolidation of laminates of these webs, particularly when laminated with disparate fibrous layers such as layers of cellulosic fibers.
Calendering of nonwovens flattens fibers and consolidates the web in a direction normal to the plane of the web and reduces the thickness. This, however, leads to reduction in permeability which is an important property to conserve for many purposes such as breathability and filtration. U.S. Pat. No. 4,048,364 discloses a process for drawing the meltblown web in the machine direction (MD) to produce a ten-fold increase in the tensile strength of the post-drawn web. It is significant to note, however, that the precursor web required in this invention contains relatively coarse fibers (10 to about 40 microns average fiber diameter) and polymer of low crystallinity. Low crystallinity generally means about 22% or less. The extensive drawing of the web reduces the diameter of the fibers in the machine direction to an average diameter of 1 to 8 microns at draw ratios ranging from 2:1 to 10:1 and preferably 5:1 to 7:1. The main purpose of the process is to increase the molecular orientation to enhance the strength of the greatly drawn fibers. Precursor webs of very high post processing draw ratio capability are required in order to prevent rupture of fibers in the web drawing process. Tests have shown that the stretching of a precursor web having hot (e.g., 10.degree. F. less than the melting point of the precursor web) drawing capabilities from about 5:1 to 10:1 does not alter the measure of pore size of the web. This is probably due to the fact that the high and easy drawability of the fibers prevents the development of sufficient, compressive forces to bend the stout fibers in the web and physically reduce its pore dimensions and measures of pore size distribution in general.
Nonwoven webs (fabrics) are defined as "sheet or web structures made by bonding and/or interlocking fibers, yarns or filaments by mechanical, thermal, chemical or solvent means." These webs do not require the conversion of fibers to yarn. Nonwoven webs are also called bonded or engineered webs and are manufactured by processes other than spinning, weaving or knitting, hence the name "nonwovens". The fibers of a nonwoven web are substantially randomly laid to form a web wherein some of the fibers are bonded by fiber-to-fiber fusion, or fiber entanglement, or thermal bonds as by point bonding. The basic structure of all nonwovens is a web of fibers or filaments. A single type of fiber or filament may be the basic element of a nonwoven. Fibers that are measured in a few centimeters or inches or fractions thereof are called staple fibers. Those fibers of extreme length are called filament fibers. In general filament fibers are measured in terms of kilometers or miles. In fact, filament fibers are not readily measured, as they may be many, many kilometers in length. In fibers, the length must be considerably greater than the diameter, e.g., a length-to-width (diameter) ratio of at least 100 and usually considerably higher. Cotton fibers may measure from less than 1/2 inch to more than 2 inches in length and have a typical length-to-diameter ratio of about 1400. Other natural fibers exhibit typical ratios as follows: flax--1200; ramie--3000; and wool--3000. In the present application, the terms "fiber" or "fibers" are intended to include both short and long fibers, i.e. staple fibers and filament fibers, unless otherwise specifically indicated by identifying the fibers as staple or filament. For example, spunbonded webs are formed of filament fibers, whereas meltblown webs include an assortment of fiber lengths so that these webs commonly include both staple length and filament length fibers. In nonwovens, the individual fibers may be in an organized or in a random arrangement. Tensile, elongation, and hand properties are imparted to the web by the type or types of bonding as well as fiber-to-fiber cohesion and reinforcement by its constituents. The technology for making nonwoven webs is based on the following primary elements: fibers of various lengths and diameters; a web arranged according to the method of forming and processing; the bonding of fibers within the web and reinforcement by its constituents. The variation of one or several elements in combination allows for the enormous range of nonwoven fiber types. Control of the type and length of the fibers and of the bonding, in combination with the selection of the manufacturing method, gives rise to a highly technical, yet extremely flexible combination of options.
Nonwoven webs of man-made, i.e. synthetic polymeric, fibers (as distinguished from "natural" fibers such as cotton, ramie, wool, etc. ) have heretofore found acceptance in the medical industry as disposable substitutes for the prior art reusable cotton examination gowns, surgical gowns, surgical drapes, face masks, shoe covers, sterilization wrap and other products, to the extent that this market for nonwoven products is estimated to exceed one billion dollars annually. Further, nonwoven webs have found use in sanitary products, such as sanitary napkins, disposable diapers, incontinent pads and other similar products. One of the benefits of nonwoven webs heretofore has been their relatively low cost, as compared to woven webs. The difference in cost between nonwoven and woven webs has heretofore been of a magnitude such that the end users can dispose of the nonwoven web product after a single use and yet realize a monetary gain over the multi-use woven webs.
Among the desired properties of a nonwoven web for use in medical and sanitary applications are the hand (softness and drapability), wicking, liquid retention, absorptive capacity and strength of the web. Also of importance in acceptance of the nonwoven web by the end user is the degree to which the nonwoven web approximates the desirable properties of the woven webs, in particularly woven cotton webs. Nonwoven webs of man-made fibers generally have the reputation of notoriously lacking many of the properties of woven natural-fiber webs, in particular hand, wicking, and liquid absorption and retention. Meltblown nonwoven webs, for example, exhibit a void volume of about 85%; spunbonded nonwoven webs exhibit a void volume of between about 90 and 95%. These webs, further, often exhibit undesirable chemical properties, such as hydrophobicity, that make the webs less than desirable for use in medical applications, for example. Moreover, the surface properties of these nonwoven webs tend to be smooth, hence exhibit a slick or oily feel and appearance. The man-made fibrous material of the prior art nonwoven webs most commonly exhibits a low surface tension so that aqueous liquids are not attracted thereto so that these prior art webs have poor wicking and retention of these liquids. These webs also are difficult to treat with liquid repellents. Still further, the filamentary nature of the man-made fibers of many prior art webs and their methods of manufacture cause the fibers to lay in the webs with the length dimension of the fibers oriented substantially parallel to the plane of the web so that the webs have poor absorbency of liquids into the body of the web. Considerable effort has been exerted heretofore to improve these properties of nonwoven webs, including modification of the manner of manufacturing and/or processing the web. These efforts, however, increase the cost of the nonwoven web and may adversely alter its monetary advantage over woven webs of natural fibers. Further, the man-made fibers of nonwoven webs most commonly are petroleum-based and therefore have been subject to the substantial fluctuations in market price of this raw material, and the important considerations in ultimate disposal of the product after use.